Dipping on Passenger side with passengers

It's time to go through the whole suspension of the car.
The first thing i would do is inspect the areas in the crossmember where the torsion bar mounts are welded on at the factory. These are notorious for not being properly welded from the factory or fatiguing over time. They are known to rust out as well.
Next, i would inspect the area in the k-member where the torsion bar goes into the hex end and the mounting rod goes through it.
These are also notorious for being weak/cracking/ovalling out. A lot of 340 and big block cars have washers welded over these holes to give it more strength.
There's no use installing the heavier duty torsion bars if there are any problems in these areas.
Then check the rear springs and spring eye bushings, as well as the rubber bushings in the rear shackles.
If everything is fixed/checks out good after that, then install the new springs, torsion bars, and i would recommend a good set of shocks and a wheel alignment if all of your front suspension is verified good.
I've seen old torsion bars break, springs break, bad bushings, etc. even on low mileage cars......
Check it all out thoroughly, at least you will have peace of mind.